1. Field of the Invention
The present invention relates to moving picture coding and decoding techniques, and in particular, to moving picture coding and decoding techniques using predictive coding of a quantization parameter.
2. Description of the Related Art
In digital moving picture coding, such as MPEG-2 Part2 (hereinafter, referred to as MPEG-2) or MPEG-4 Part10/H 0.264 (hereinafter, referred to as AVC), each picture is partitioned into blocks having a predetermined size and coding is performed, and a quantization parameter indicating the roughness of quantization for a prediction error signal (or simply a picture signal) is transmitted. By variably controlling the quantization parameter in units of a predetermined block on the coding side, it is possible to control the code amount or to improve the subjective picture quality.
As quantization parameter control for improving the subjective picture quality, Adaptive Quantization is often used. In the adaptive quantization, change according to the activity of each macroblock is made such that quantization is applied more finely in a flat portion, in which deterioration is visually easily recognized, and quantization is applied more coarsely in a complex design portion, in which the visual recognition of deterioration is relatively difficult. That is, in a macroblock with high activity for which the assigned bit amount at the time of coding tends to increase, the quantization parameter is changed so that the large quantization scale is set. As a result, the subjective picture quality is improved while performing control such that the number of bits in the data of the coded picture is reduced as much as possible.
In the MPEG-2, it is determined whether or not the quantization parameter of the last block in coding/decoding order is the same as the quantization parameter of a block to be coded, and the quantization parameter is transmitted when the quantization parameter of the last block is not the same as the quantization parameter of the block to be coded. In the AVC, differential coding of the quantization parameter of the block to be coded is performed using the quantization parameter of the last block in coding/decoding order as a predicted value. This is based on the fact that code amount control is performed in coding order in general, and accordingly, the quantization parameter of the last block in coding order is closest to the quantization parameter of the coding block, and the purpose is to suppress the amount of information of the quantization parameter to be transmitted.